1. Field of the Invention
The present invention relates to an electric waveform generating circuit for a small brushless motor, and, in particular, to an electric waveform generating circuit for a brushless motor wherein the amount of noise accompanying the electromagnetic noise and rotation of the motor is small, there are few externally-connected parts, and the number of internal circuit structural elements thereof is greatly reduced.
2. Description of the Prior Art
Conventionally, in controlling the speed of a small brushless motor the relative positions of the field pole of the rotor and the armature winding are detected by means of position sensors, and the speed is controlled by a control circuit based on the detected positions.
In such a case it is difficult to provide position sensors with uniform characteristics to a motor, and the difference in the characteristics of the position sensors causes a problem by adversely affecting the operating characteristics of the position sensors under high speed rotation.
Various sensorless methods for driving the motor have been proposed to eliminate these problems in the prior art described above. The Phase-Locked Loop (PLL) method, which provides a phase detector and a voltage controlled oscillator, and a filter method are examples.
The speed of a conventional small brushless motor is mainly controlled by means of a hardware switching method using a switching element such as a transistor or the like.
When the current of the motor coil in a motor is changed digitally by the hardware-switching method, there is a tendency to produce magnetic noise and noise from the motor itself which is a problem.
With an apparatus which is adversely affected by noise, in particular in a motor used for a video camera set, a soft-switching method which provides analogue control of the commutation waveform is preferable.
There has recently been a demand for provision of a motor drive method using the sensorless and soft-switching method but there have been few cases of commercialization of such methods. Even when the method is commercialized, a large number of externally-mounted parts is required, or large-scale circuitry is necessary for digital processing.
As outlined above, with a circuit for generating electric waveforms for a conventional brushless motor,
(1) the hardware-switching shows a tendency to produce magnetic noise and noise from the motor itself; PA1 (2) in the soft-switching method which eliminates the problems of (1), a large number of externally-mounted parts is required to provide a brushless motor based on the soft-switching method, or large-scale circuitry is necessary for digital processing. PA1 variable frequency generating means for generating a triangular wave; PA1 inverting means for inverting the triangular wave output by the variable frequency generating means; PA1 analogue switch means for opening and closing based on a specified timing signal synchronized with the oscillating cycle of the triangular wave from the variable frequency generating means for synthesizing the triangular wave and the reversed wave; and PA1 synthesizing means for outputting a synthesized wave form from the output of the analogue switch means. PA1 variable frequency generating means for generating a triangular wave; PA1 pulse generating means for generating a specified timing signal synchronized with the oscillating cycle of the triangular wave from the variable frequency generating means; PA1 inverting means for inverting the triangular wave output by the variable frequency generating means; PA1 group of analogue switches which are opened and closed based on the control of the timing signal for synthesizing the triangular wave and the reversed wave; and PA1 synthesizing means for outputting a synthesized wave form from the output of at least one of the switches in the analogue switch group.